This is not possible, but also usually not needed.
Decisions like this are usually made because partitions get too big to be backed-up regularly, or because the customer wants to have a feeling where the data is located, or wants to delete data by hand.
If the issue is the backup size, you can close your partition and create a new one, combined with only backing up the closed partitions one a month/quarter. All new items will land in the new, small and open partition.
If the issue is keeping tabs on the data, Enterprise Vault will know where the data is at. Just ask, and it will serve.
If the issue is the possibility to delete by hand, DO NOT DO THIS. It will cause database inconsistencies resulting in strange behaviour towards end-users. Better solution is to expire by setting the correct retention poliicy, expire the data and let EV delete it for you.
If there's something else causing the need to split off the data, please, let us know!